1. Field of the Invention
The invention relates to an air-fuel ratio control apparatus and an air-fuel ratio control method for an internal combustion engine.
2. Description of the Related Art
The air-fuel ratio in an internal combustion engine must be accurately controlled for an exhaust gas control catalyst to be able to effectively purify the exhaust gas. In order to control the air-fuel ratio, the amount of fuel to be injected is calculated based on the intake air amount detected by an airflow meter or the like. Furthermore, the air-fuel ratio is also feedback-controlled by adjusting the fuel injection quantity based on the output of an air-fuel ratio sensor arranged in the exhaust passage.
The air-fuel ratio control described above does enable the air-fuel ratio of the overall internal combustion engine to be accurately controlled. However, even though the desired air-fuel ratio for the overall internal combustion engine can be obtained, when looking at the cylinders individually, air-fuel ratio variation occurs between cylinders due to differences in, for example, the intake air characteristics and the injection characteristics of the fuel injection valves.
If there is air-fuel ratio variation between cylinders, exhaust emissions deteriorate even if the air-fuel ratio for the overall internal combustion engine is the stoichiometric air-fuel ratio. Also, if there is air-fuel ratio variation between cylinders, the torque generated in each cylinder will be different, which may lead to torque fluctuation. Thus, it is desirable to detect and correct any air-fuel ratio variation between cylinders. When there is air-fuel ratio variation between cylinders, if the air-fuel ratio variation is small, the air-fuel ratio variation can be corrected by an air-fuel ratio feedback control, and a catalyst can purify pollutant components in exhaust gas, and therefore, a problem is not caused. However, when the air-fuel ratio variation between the cylinders is large, for example, due to a malfunction of a fuel injection system for a part of the cylinders, exhaust emissions deteriorate, and a problem is caused. It is preferable that the large air-fuel ratio variation that deteriorates the exhaust emissions should be detected as abnormal air-fuel ratio variation. Particularly, it is required to detect the abnormal air-fuel ratio variation between the cylinders in the internal combustion engine mounted in the vehicle, to prevent the vehicle from traveling when exhaust emissions from the vehicle deteriorate. Recently, there has been a movement for making it mandatory to detect the abnormal air-fuel ratio variation. Accordingly, when there is abnormal air-fuel ratio variation between the cylinders, it is preferable to detect the abnormal air-fuel ratio variation between the cylinders.
One conceivable method for detecting air-fuel ratio variation between cylinders is to arrange an air-fuel ratio sensor that detects the exhaust gas air-fuel ratio in each cylinder. Employing this method, however, greatly increases costs as it requires the same number of air-fuel ratio sensors as there are cylinders.
Japanese Patent No. 2689368 describes an apparatus which provides a single wide range air-fuel ratio sensor in a merging portion in the exhaust system, models the time that it takes (i.e., delay) for the air-fuel ratio sensor to detect the exhaust gas discharged from each of the cylinders, and estimates the air-fuel ratio of each cylinder by an observer.
According to the apparatus that estimates the air-fuel ratio of each cylinder described in Japanese Patent No. 2689368 above, the air-fuel ratio of each of a plurality of cylinders can be estimated with a single air-fuel ratio sensor. However, there are various limitations when it comes to employing the apparatus described in that publication.
One such limitation is that it requires that the gas transfer delay from each cylinder to the air-fuel ratio sensor be a constant delay. Therefore, the length of the exhaust manifold must be uniform for each cylinder. Designing an actual exhaust manifold shape so that it will satisfy this kind of limitation is difficult. In particular, making the length of the exhaust manifold uniform for each cylinder in a V-type engine is structurally near impossible.
Another limitation is that the exhaust gas from each cylinder must pass through the air-fuel ratio sensor in a state in which it is, to the greatest extent possible, not mixed with the exhaust gas from other cylinders. Therefore, the location where the air-fuel ratio can be mounted is limited to the merging portion (joining portion) in the exhaust system.
A third limitation is that the air-fuel ratio sensor must be sensitive to the exhaust gas coming from each cylinder that flows at extremely short intervals of time. That is, the air-fuel ratio sensor must be have extremely good (i.e., fast) responsiveness.
Various limitations such as those described above make it extremely difficult in actuality to adapt the apparatus that estimates the air-fuel ratio of each cylinder described in the foregoing publication.